1. Field of the Invention
The invention relates generally to the field of marine seismic data acquisition and processing. More particularly, the invention relates to methods for processing marine seismic signals to attenuate the effects of certain types of noise.
2. Background Art
Seismic surveying is known in the art for determining structures of rock formations below the earth's surface. Seismic surveying generally includes deploying an array of seismic sensors at the surface of the earth in a selected pattern, and selectively actuating a seismic energy source positioned near the seismic sensors. The energy source may be an explosive, a vibrator, or in the case of seismic surveying performed in a body of water such as the ocean, one or more air guns or water guns.
Seismic energy which emanates from the source travels through the earth formations until it reaches an acoustic impedance boundary in the formations acoustic impedance boundaries typically occur where the composition and/or mechanical properties of the earth formation change. Such boundaries are typically referred to as “bed boundaries.” At a bed boundary, some of the seismic energy is reflected back toward the earth's surface. The reflected energy may be detected by one or more of the seismic sensors deployed on the surface. Seismic signal processing known in the art has as one of a number of objectives the determination of the depths and geographic locations of bed boundaries below the earth's surface. The depth and location of the bed boundaries is inferred from the travel time of the seismic energy to the bed boundaries and back to the sensors at the surface.
Seismic surveying is performed in the ocean and other bodies of water (“marine seismic surveying”) to determine the structure and composition of rock formations below the sea bed. Marine seismic surveying systems known in the art include a vessel which tows one or more seismic energy sources, and the same or a different vessel which tows one or more “streamers.” Streamers are arrays of seismic sensors in a cable that is towed by the vessel. Typically, a seismic vessel will tow a plurality of such streamers arranged to be separated by a selected lateral distance from each other, in a pattern selected to enable relatively complete determination of geologic structures in three dimensions. It is also known in the art to place cables having seismic sensors (“ocean bottom cables”) along the sea bed, and actuate a seismic energy source in the water. Typically, the seismic energy source will be towed by a vessel just as in streamer-type surveying.
In addition to reflected seismic energy both coherent noise and incoherent noise may be present in the detected seismic energy. The presence of noise in the energy detected by the seismic sensors reduces the signal to noise ratio (“SNR”) of the seismic signals of interest. One objective of seismic data processing is, therefore, reduce the effects of noise on the signals detected by the seismic sensors without appreciably reducing the true seismic signal component of the detected signals.
Prior art methods which have been used to reduce the effects of noise and acquire a higher quality representation of a particular subsurface structure include using multiple actuations of the seismic source (multiple “firings” or “shots”) to record a plurality of sensor measurements from substantially the same subsurface structure, and then summing or “stacking” such measurements to enhance signal strength while substantially reducing the effects of random or incoherent noise.
U.S. Pat. No. 5,818,795 which is assigned to an affiliate of the assignee of the present invention, and which provides a detailed summary of prior art methods and systems addressing the problem of noise suppression in seismic signals, discloses a method of reducing the effect of “burst” noise in seismic signal recordings without eliminating seismic signals of interest.
U.S. Pat. No. 5,761,152, which is assigned to an affiliate of the assignee of the present invention, describes a method and system for marine seismic surveying. The method disclosed in the '152 patent includes increasing the fold (number of recorded reflections from a same reflector), and as a result the signal-to-noise ratio of coherent seismic signals, without incurring the problems of drag, entanglement, complicated deck handling associated with increased streamer length, increased number of streamers, and increased distance between streamers. Source and streamer “offsets”, and time of firing of lead and trailing vessel sources in a time delay sequence are optimized to increase the fold while avoiding substantial influence by the seismic signals resulting from the source of one vessel on the seismic signals resulting from the source of the other vessel.
The foregoing description is not meant to be an exhaustive explanation of the types of noise and the methods for reducing the effects thereof in seismic signals. There are two types of noise, however, for which prior art methods have not reduced the effects to an acceptable degree. One of these types of noise is generated by vessels or rigs in the vicinity of the seismic vessel (here termed “continuous ship noise”). The other type of noise is impulsive noise which originates as a result of actuation of the seismic energy source, wherein the seismic energy travels laterally through the water both directly to the sensors, and to reflectors in the water and then back to the sensors on the array (the latter being termed “back scattered noise”). Such impulsive noise may also originate from the seismic energy sources used by a different seismic acquisition system located and operated some distance from the acquisition system in question. It is desirable to have a method for attenuating impulsive noise and continuous ship noise in marine seismic data. In the case of noise generated by another seismic system, one technique known in the art is “time sharing” wherein data acquisition by one system is delayed during data acquisition by the other system. Such time sharing reduces operating efficiency of on both systems.
U.S. Pat. No. 6,751,559 issued to Fookes et al. and assigned to an affiliate of the present invention describes a method for attenuating noise from marine seismic signals caused by a noise in the water. The disclosed method includes determining an arrival time of a noise event at each of a plurality of seismic sensors, estimating a position of the noise source from the arrival times, and attenuating the noise event from the signals detected by the seismic sensors.
There continues to be a need for improved techniques for attenuation of noise in seismic data.